Various combustor systems are well known in gas turbine applications to reduce the creation of pollutants in the combustion process. As known, gas turbines include a compressor for compressing air, a combustion stage for producing a hot gas by burning fuel in the presence of the compressed air produced by the compressor, and a turbine for expanding the hot gas to extract shaft power. The combustion process in many older gas turbine engines is dominated by diffusion flames burning at or near stoichiometric conditions with flame temperatures exceeding 3,000° F. Such combustion, however, typically produces a high level of oxides of nitrogen (NOx). Current emissions regulations have greatly reduced the allowable levels of NOx emissions.
One method for reducing combustion temperatures is to provide a lean, premixed fuel to the combustion stage. In a premixed combustion process, fuel and air are premixed in a premixing section of the combustor. Swirling may be induced to improve mixing as described in U.S. Pat. No. 6,082,111, and incorporated herein by reference. The fuel-air mixture is then introduced into a combustion stage where it is burned. Accordingly, local fuel-air ratios can be kept low enough so that flame temperatures are below those that produce substantial NOx emissions. However, the difficulty with lean, premixed combustion is that the lean flames may be unstable, and additional steps may be necessary to ensure that the flame remains stable.
One method of stabilizing a lean flame is to provide a stable, high temperature diffusion flame as a pilot flame to provide a constant source of ignition for the lean fuel-air mixture. A portion of the fuel and air supplied to the combustor is reserved to provide for the pilot flame. However, a diffusion flame is a source of NOx and, consequently, the size of the pilot flame must be minimized, such as by premixing the fuel and air provided to the pilot flame, to decrease NOx emission. In addition to pilot flame optimization, the degree of mixing of the fuel and air can minimize formation of NOx pollutants. This approach can produce NOx levels as low as 6 to 9 parts per million (ppm) if well engineered, but stability of the lean flame is still a concern.
Another method to reduce NOx emissions is to use a Rich-Quench-Lean (RQL) technique, wherein a rich fuel air mixture is ignited and partially combusted before being quickly diluted with an injection of air to create a lean mixture. However, it is difficult to achieve rapid, uniform mixing of the injected air with the partially combusted rich fuel air mixture to quickly drive the overall mixture to a lean state while avoiding high temperature quasi-diffusion flame zones.
In yet another method to reduce NOx emissions, catalytic combustion can be used to stabilize the lean premixed flame instead of using a pilot flame. In one approach, the bulk lean mixture can be passed through a catalyst combustor section wherein a catalytic material (for example, a noble metal such as platinum or palladium) is adhered to a metal substrate. In this lean catalytic approach, the mixture is partially converted before exiting the catalytic section and is raised in temperature so that the catalyzed mixture burns stably downstream. The problem with this approach is that the lean mixture must be somewhat preheated (a step that generates NOx emissions) to be ignited by the catalyst, and it is possible to allow the catalytic reaction to proceed too far, thus exposing the catalyst to damaging temperatures. In addition, the catalytic combustor section is expensive and requires increased servicing and replacement. If well engineered, this approach can produce NOx levels as low as 2 to 4 ppm, but optimal mixing is required.
Yet another method to reduce NOx emissions is to pass a rich reactive mixture of the fuel and a relatively small portion of air over a set of catalyst coated tubes or plates to form a high temperature fuel gas. Such a system is described in U.S. Pat. No. 6,415,608, owned by the assignee of the current invention and incorporated herein by reference. The coated tubes or plates are cooled by a remaining larger portion of the air provided to the combustor by passing the larger portion of air over the non-catalytic backsides of the tubes or plates in a “backside cooling” configuration. This technique has advantages over the lean catalytic method because the catalyst is less prone to overheating, and no preheating of the fuel-air mixture is required. By nature of the tube discharge configuration, this technique provides enhanced premixing of the high temperature fuel gas and the larger portion of the air in a downstream homogenous burnout zone. This approach can produce NOx levels as low as 1 to 3 ppm, if well engineered.
There is an ongoing need for improved combustion techniques to provide low NOx emissions and stable combustion conditions.